1. Field of the Invention
The present invention related to the field of current mirrors as commonly used in integrated circuits.
2. Prior Art
Current mirrors are very frequently used in integrated circuits to set bias currents for various parts of the circuit. Typically the currents of one or more current sources, such as a current source that is independent of temperature or proportional to absolute temperature, is mirrored to various parts of a circuit so that one (or a very few) current sources may be mirrored to numerous sub-circuits as local current sources for biasing purposes.
Conventional current mirrors are comprised of an input and an output MOS/junction transistor having their sources/emitters connected to one power supply rail and their gates/bases connected together and to the drain/collector of the input transistor. Applying a current to the drain/collector of the diode connected input transistor sets the gate/base voltage of the input transistor and thus the gate/base voltage of the output transistor, biasing the output transistor so that its drain/collector current will be approximately proportional to the current in the drain/collector of the input transistor. However, for a junction transistor current mirror, the collector of the input transistor also carries the base current of both transistors, limiting the accuracy of the current mirror, and the Early effect limits the output impedance of the output transistor. At low voltage operation of both MOS and junction transistor current mirrors, the drain/collector current in the output transistor is even more dependent on the drain/collector voltage, resulting in the output impedance of the current mirror being both low and voltage dependent.
Low voltage, high impedance current mirrors realizable in MOS or junction transistor circuits and particularly suited for use in integrated circuits. The current mirrors use first and second transistors coupled as a differential pair with a tail current that may be part of the input current to be mirrored. Another component of the input current to be mirrored is applied to the drain/collector of the first transistor of the differential pair, with the gate/base of that transistor being coupled to a bias voltage. The voltage on the drain/collector of the first transistor is effectively inverted and used to control the gate/base of the second transistor to provide a drain/collector current in the second transistor equal to the difference between the tail current and the current in the drain/collector of the first transistor. Various embodiments are disclosed, including an embodiment using a simple current mirror to provide the tail current for the differential pair.